Question

\(\mathrm{CO}_{2}\) deprivation. An illuminated suspension of Chlorella is actively carrying out photosynthesis in the presence of \(1 \% \mathrm{CO}_{2}\) The concentration of \(\mathrm{CO}_{2}\) is abruptly reduced to \(0.003 \%\). What effect would this reduction have on the levels of 3 -phosphoglycerate and ribulose 1,5 -bisphosphate in the next minute?

Short answer

3-phosphoglycerate levels decrease, ribulose 1,5-bisphosphate accumulates.

Step-by-step solution

Understand Photosynthesis

Photosynthesis involves two main stages: the light-dependent reactions and the Calvin cycle. In the Calvin cycle, carbon dioxide (CO2) is fixed by the enzyme RuBisCO to form 3-phosphoglycerate (3-PGA) as an initial product.

Identify the Role of CO2

CO2 is a substrate for the enzyme RuBisCO in the Calvin cycle, where it combines with ribulose 1,5-bisphosphate (RuBP) to eventually form 3-PGA. A reduction in available CO2 will directly affect this reaction.

Analyze Effect on 3-Phosphoglycerate Levels

With a significant decrease in CO2 concentration from 1% to 0.003%, the rate of the initial carboxylation reaction in the Calvin cycle decreases. This leads to a reduction in the production of 3-PGA.

Determine the Consequence for Ribulose 1,5-Bisphosphate

As 3-PGA production decreases, ribulose 1,5-bisphosphate (RuBP) accumulates. This is because RuBP is not being converted efficiently into 3-PGA due to the lack of CO2 available for the carboxylation step.

Predict Overall Impact

In the context of photosynthesis, a sharp decrease in CO2 levels would lead to an accumulation of RuBP and a decrease in 3-PGA within the next minute following the reduction in CO2 concentration.

Key concepts

Calvin cycle

The Calvin cycle is an essential part of photosynthesis that occurs in the stroma of chloroplasts. During this cycle, plants convert carbon dioxide and other compounds into glucose, which serves as an energy source.
Unlike the light-dependent reactions, which require sunlight, the Calvin cycle is a light-independent process. It can occur both in the presence and absence of light.

• This cycle consists of three main stages: carbon fixation, reduction, and regeneration.
• Carbon fixation involves the enzyme RuBisCO, which facilitates the incorporation of CO$_{2}$ into an organic molecule.
• In the reduction phase, ATP and NADPH produced in the light-dependent reactions are used to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P).
• Finally, in the regeneration stage, some G3P molecules are used to regenerate ribulose 1,5-bisphosphate (RuBP), allowing the cycle to continue.

This cycle is vital for photosynthesis, as it ultimately leads to the production of glucose from carbon dioxide and water.

ribulose 1,5-bisphosphate

Ribulose 1,5-bisphosphate (RuBP) is a critical molecule in the Calvin cycle. RuBP acts as the carbon dioxide acceptor in the carbon fixation process.
RuBP is a five-carbon sugar phosphate that becomes essential during the initial reaction of the Calvin cycle.

• It combines with carbon dioxide, catalyzed by the enzyme RuBisCO, to form a six-carbon intermediate.
• This intermediate is then split immediately into two molecules of 3-phosphoglycerate (3-PGA).
• The availability of RuBP is crucial for the photosynthesis process to proceed efficiently.

In the absence of sufficient CO$_{2}$, RuBP accumulates because it is not converted into 3-PGA, affecting the rate of the Calvin cycle overall.

3-phosphoglycerate

3-Phosphoglycerate (3-PGA) is one of the first stable products formed in the Calvin cycle. This three-carbon molecule plays a pivotal role in synthesizing carbohydrates from CO$_{2}$.
The creation of 3-PGA marks the entry of carbon into the biosphere via photosynthesis.

• When RuBP reacts with carbon dioxide, catalyzed by RuBisCO, it results in the formation of 3-PGA.
• The abundance of 3-PGA depends directly on the availability of CO$_{2}$.
• As CO$_{2}$ levels drop, like in high reductions, the production of 3-PGA decreases sharply.
• 3-PGA is subsequently converted, through a series of reactions, into glucose and other sugars, providing energy for the plant.

Overall, 3-PGA is crucial for converting inorganic carbon into organic molecules through the Calvin cycle.

RuBisCO

RuBisCO, formally called ribulose-1,5-bisphosphate carboxylase/oxygenase, is an enzyme that plays a fundamental role in the process of photosynthesis.
It is one of the most abundant enzymes found on earth, thanks to its critical function in carbon fixation during the Calvin cycle.

• RuBisCO facilitates the reaction between RuBP and CO$_{2}$, resulting in the formation of 3-PGA.
• Despite its abundance, RuBisCO is inefficient and slow, which is why large quantities of it are needed to maintain adequate photosynthesis rates.
• This enzyme also has a dual activity - it can fix oxygen instead of carbon dioxide, leading to a process called photorespiration, which is less efficient.

Without RuBisCO, the Calvin cycle cannot operate, which halts the production of the crucial carbohydrates needed for plant growth and energy storage.